Medulloblastoma (?) with epithelioid features.

A midline cerebellar tumor in an 18-year-old male, on light microscopy composed of closely packed, small featureless cells with little cytoplasm, under electron microscopic examination showed distinct epithelial features. The cells were closely apposed and joined by zonula adherentes, puncta adherentia and classical desmosomes. The latter structure has not been described in unequivocal glial or neuronal tumors or in normal cells derived from the neural tube in vertebrates, not even in ontogenesis. In many areas the tumor cells abut on extensive basement membranes. Although the majority of the cells were undifferentiated, we occasionally observed elements containing dense core vesicles, microtubules and cilia. In addition, typical astrocytes were identified. The presence of these features strongly favors the neuroectodermal origin of this neoplasm.